Lawyers for three mental health patients at Bridgewater State Hospital filed a class-action lawsuit against the facility Thursday, arguing that more than half of the patients there are being wrongly held in “harsh conditions of confinement,” even though they are not serving criminal sentences.

The lawsuit, filed in Norfolk Superior Court, calls for the transfer of approximately 175 patients locked up at Bridgewater mainly because they can be violent, not because they have been convicted of crimes. The lawsuit argues that the men need treatment, not incarceration.

Massachusetts is one of the few states that house civilly committed mental health patients — including those referred for psychiatric evaluations to determine whether they are competent to stand trial — in facilities run by the prison system. Most states house such patients in hospitals run by a department of mental health.

The mental and physical health of many Bridgewater patients is deteriorating, the lawsuit asserts, because the hospital illegally isolates patients, sometimes strapping them to beds. The suit says the hospital delivers inadequate mental health care “that would shock the conscience of a reasonable person.”

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The mother of one of the three plaintiffs said she saw the class-action lawsuit as the only way to help her son, who has been diagnosed with autism and schizophrenia and has been held at Bridgewater since 2013, often in complete isolation.

“The system would just process and crush him, and he’d be there forever, secluded forever,” said the mother, who asked to remain anonymous to protect her family’s privacy. Her son is identified only as John Doe in the lawsuit.

Officials in Governor Deval Patrick’s office did not respond to the lawsuit’s allegations directly, but stressed that state officials want humane care for people suffering from mental illness. The Department of Correction said it puts patients in seclusion for clinical reasons.

“Governor Patrick has been clear that seclusion and restraint should only be used for the most exceptional situations and only as a measure of last resort to keep individuals from harming themselves or others,” said Heather Nichols, Patrick’s press secretary. “We are committed to ensuring individuals suffering from mental illness are provided the appropriate treatment in the appropriate settings.”

Bridgewater State Hospital is a prison accredited as a behavioral health care provider, not a hospital, and it is ill-equipped to deliver adequate mental health care, especially in cases in which patients are severely mentally ill, the lawsuit says. State statistics show that Bridgewater patients are placed in restraint or isolation more than 100 times as often as patients at state mental health facilities.

The lawsuit follows a series of reports in the Globe citing questionable use of seclusion and restraints at Bridgewater, including the events leading up to the 2009 death of Joshua Messier, a 23-year-old mental health patient who died while guards were strapping his wrists and ankles to a small bed. The medical examiner ruled Messier’s death a homicide, but the prosecutor declined to press charges, and no one was disciplined until the Globe’s February article.

Since the Globe story, Patrick has disciplined or fired six correction officials connected to Messier’s death, and a watchdog group, the Disability Law Center, has begun an investigation of conditions at the facility.

The three mental health patients whose families brought the lawsuit include the 32-year-old patient diagnosed with autism and schizophrenia, whose parents have asked for anonymity; Felipe Zomosa, a 31-year-old Peabody man diagnosed with schizophrenia and bipolar disorder; and Peter Minich, a 31-year-old Brookline man diagnosed with paranoid schizophrenia whose parents filed an earlier lawsuit on his behalf.

According to the class-action lawsuit, all three were referred to Bridgewater for psychiatric evaluations after being accused of assaulting staff members at psychiatric facilities. All three were not convicted of a crime; and all three “have been repeatedly secluded and placed in four- and five-point mechanical restraints on numerous occasions in violation of state law,” which limits such tactics to life-threatening emergencies.

The lawsuit also says that Minich and Zomosa “have spent more time in seclusion and restraint in one year than all 626 patients in state [Department of Mental Health] facilities combined over the same period.”

‘It’s very disturbing how someone who is so disabled could be sent to prison.’

Daniela Zomosa, Felipe’s sister, said in an interview that her brother entered the US Army after graduating from high school and was discharged several years ago because of mental health problems.

“He said he was hearing voices and had really bad mood swings,” she said.

As his mental illness grew worse, she said, he was admitted to a Lynn psychiatric hospital. But last April he was accused of assaulting a psychiatrist and, after criminal charges were filed, was referred to Bridgewater for an evaluation. He has been kept there since then, being deemed too dangerous to be released or transferred to a DMH facility.

Now, Daniela said, she and other members of her family, including her brother and her mother, would like Felipe released to a psychiatric hospital for treatment, then returned to the family’s Peabody home.

“He’s missed a whole year with us, holidays and everything,” she said.

The mother of the patient diagnosed with autism and schizophrenia said her son was a patient in the psychiatric unit at Lemuel Shattuck Hospital in Jamaica Plain for most of a decade until last year, when staff members were preparing to transfer him to a group home.

“He was doing well in cooking class and music,” she said.

But last November her son was accused of punching a psychiatrist, she said, perhaps because the psychiatrist had reduced the dosage of his psychotropic medications, and was referred to Bridgewater for a psychiatric evaluation, where he remains.

“It’s very disturbing how someone who is so disabled could be sent to prison,” said the mother. “It seems ridiculous. It seems medieval.”

Minich, a 31-year-old Brookline resident and Wellesley High School graduate, was also referred to Bridgewater for a psychiatric evaluation after allegedly assaulting clinicians at Shattuck Hospital. Last month, his parents signed an agreement with Bridgewater officials that has significantly improved his treatment, said Roderick MacLeish Jr., a lawyer with the firm of Clark, Hunt, Ahern & Embry and lead attorney in the Bridgewater case.

Officials at the Department of Correction declined to comment on treatment of the three plaintiffs, but said: “The use of seclusion and restraint at Bridgewater State Hospital is a clinical decision and one we view as a measure of last resort. These clinical decisions are based on an individual’s specific mental health treatment needs.”

Defendants in the lawsuit, in addition to the Department of Correction and the Department of Mental health, include Correction Commissioner Luis S. Spencer and Robert Murphy, superintendent at Bridgewater. Both Spencer and Murphy were reprimanded by Patrick earlier this year for the roles they played in the aftermath of Messier’s death.

The lawsuit also named Mental Health Commissioner Marcia Fowler as a defendant, in part because staff members at Department of Mental Health facilities have been filing criminal charges against mental health patients and getting them transferred to Bridgewater, despite a 2001 policy that discourages the practice. Moreover, MacLeish contends, the transfer of mentally ill people to a prison due to behavior they cannot help amounts to a “bad faith” violation of a 1988 agreement that MacLeish himself negotiated with the state.

Under terms of the settlement, the Department of Mental Health opened a unit for mentally ill patients who can become violent but shuttered the unit at Taunton State Hospital in 2003. Since that time, officials have increasingly sent mentally ill men to Bridgewater instead, the lawsuit says.

The Executive Office of Human Services issued a statement Thursday night defending the care provided by the Department of Mental Health.

“The Department of Mental Health is a national leader in behavioral health, and we are committed to providing quality, safe, evidence-based care and services to individuals in our inpatient and community systems,” the statement said.

“It is clear that the systemic problems at Bridgewater State Hospital have been publicly known for a long period of time, and we are going to undertake vigorous efforts to determine what knowledge the governor had,” MacLeish said.

MacLeish will ask a judge to declare the lawsuit a class action. The proposed class would include most of those who are not serving criminal sentences, such as those initially referred to the facility for psychiatric evaluations and then committed for longer periods.

But the proposed class does not include convicted criminals who have been referred to Bridgewater by other state prisons and county houses of correction, patients found not guilty by reason of insanity, or patients who have completed prison terms and remain at Bridgewater because they are still considered dangerous.

Noting a parallel crisis in the state’s Department of Children and Families following the recent deaths of three children, MacLeish said: “It seems that the social contract between our government and our most vulnerable citizens, whether children or severely disabled adults, has been broken. It’s our responsibility to fix it.”

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